The present invention relates to a paint composition for automotive weather strips and glass runs, as well as processes for producing automotive weather strips and glass runs.
Most automotive weather strips and glass runs (which are hereunder sometimes referred to simply as xe2x80x9cautomotive weather stripsxe2x80x9d) are coated with curable urethane-based paints on the surface of the substrate to impart it special functions such as wear resistance and slip property (sliding property) or provide it with better appearance.
Conventionally, curable urethane-based paints of a solvent type have been used in the manufacture of automotive weather strips; however, with the recent concern over the global environment and the health of working personnel, a need has arisen for a shift toward aqueous and other paints of a non-solvent type.
However, water which is the medium for paints of the aqueous type have more latent heat of evaporation than solvents, so the aqueous paints require more heat than paints of the solvent type in order to dry up the water after application. In addition, unlike solvent-type paints that allow adjustment of volatilization temperature and rate by combining several kinds of solvents, aqueous-type paints require preliminary drying in order to prevent xe2x80x9cflashingxe2x80x9d due to nearly instantaneous volatilization of water. As a result, longer drying times are required by the aqueous paints and in order to deal with this low productivity problem, it becomes necessary to build a new drying oven for shifting from the solvent-type paint to the aqueous type.
Most automotive weather strips use substrates that are made of thermosetting elastomers such as EPDM rubber (ethylene propylene rubber) or thermoplastic elastomers such as TPO (thermoplastic polyolefins).
If thermoplastic elastomers are used as the substrate, drying should be carried out at a low temperatures of 150xc2x0 C. or below to prevent thermal deformation that would otherwise occur during post-application drying; however, if the paint applied is of the aqueous type, the drying operation is not highly productive since water is very difficult to dry.
The EPDM rubber as an exemplary thermosetting elastomer is nonpolar and has low sticking property and it is also hydrophobic; hence, aqueous paints cannot produce a more adhesive coat than solvent-type paints.
With a view to solving this problem, pretreatments such as corona discharge and primer application are conventionally applied to the substrate surface but problems still remain, such as high initial cost and the difficulty involved in performing positive pretreatments on complexly shaped articles.
Therefore, a first object of the invention is to provide a paint for automotive weather strips that is free from the above-mentioned defects of the prior art and which can achieve strong adhesion to the EPDM rubber without corona discharge, primer application or other pretreatments on the substrate surface while exhibiting high wear resistance.
A second object of the invention is to provide a process for producing automotive weather strips which does not require the as-applied coat to be cured completely in a drying oven but which permits it to be cured completely by the heat inertia of the drying step and which can also shorten the length of the drying oven or lower the drying temperature.
A third object of the invention is to provide a process by which automotive glass runs having high wear resistance in the bottom portion while exhibiting high softness and flexibility in the lip portions can be produced efficiently and with minimum impact on the global environment.
As a result of the extensive studies made in order to attain the above-mentioned objects, the present inventors found that the first object of the invention could be attained by adding at least two specified silane coupling agents or a product of premixing reaction between said at least two silane coupling agents to a curable urethane-based emulsion paint.
Thus, in a first aspect, the present invention relates a paint composition for automotive weather strips comprising a curable urethane-based emulsion paint having added thereto either at least two silane coupling agents selected from the group consisting of a silane coupling agent having an amino group, a silane coupling agent having an epoxy group, a silane coupling agent having a methacryloxy group and a silane coupling agent having an acryloxy group, or a reaction product obtained by previously mixing said at least two silane coupling agents.
The second object of the invention can be attained by a process for producing automotive weather strips which comprises the steps of extruding a semi-finished product of automotive weather strip while it is continuously coated with a urethane-based aqueous paint, then drying and curing the product in a heating furnace, or comprises extruding a semi-finished product of automotive weather strip, heating it, immediately followed by continuous application of a urethane-based aqueous paint, then drying and curing the product in a heating furnace, or comprises unrolling a semi-finished extruded product of automotive weather strip while it is continuously coated with a urethane-based aqueous paint, then drying and curing the extruded product in a heating furnace, wherein a silicone compound having an amino group is applied to the coated surface of the semi-finished product or extruded product after it leaves the heating furnace, or wherein a silicone compound having an amino group is incorporated in the urethane-based aqueous paint.
The third object of the invention can be attained by a process for producing automotive glass runs which comprises the steps of extruding a semi-finished product of automotive glass run as while is continuously coated with a silane-crosslinkable polyethylene, then drying and curing the product in a heating furnace, or comprises the steps of extruding a semi-finished product of automotive glass run, heating it, immediately followed by continuous coating with a silane-crosslinkable polyethylene, then drying and curing the product in a heating furnace, or comprises the steps of unrolling a semi-finished extruded product of automotive glass run while it is continuously coated with a silane-crosslinkable polyethylene, each of which processes further comprises applying an urethane resin-based aqueous paint containing a silicone compound having an amino group or a silicone compound having an amino group or a solution thereof onto the silane-crosslinkable polyethylene coat, then heating the applied product in a heating furnace to dry and cure the applied aqueous paint, silicone compound or solution and to crosslink the silane-crosslinkable polyethylene.